1. Technical Field of the Invention
The present invention relates generally to spark plugs for internal combustion engines. More particularly, the invention relates to a spark plug with an improved structure in which a metal shell has a threaded portion with an outer diameter of 12 mm or less. The improved structure ensures the spark plug of high insulation properties and a high capability to ignite the air-fuel mixture (referred to as ignition capability hereinafter).
2. Description of the Related Art
Conventional spark plugs for use in internal combustion engines generally include a tubular metal shell, an insulator, a center electrode, and a ground electrode.
The metal shell has a threaded portion for fitting the spark plug into a combustion chamber of the engine. The insulator has a center bore formed therein and is fixed in the metal shell such that an end thereof protrudes from an end of the metal shell. The center electrode is so secured in the center bore of the insulator that an end thereof protrudes from the end of the insulator. The ground electrode has a tip portion and is joined to the end of the metal shell such that the tip portion faces the end of the center electrode through a spark gap therebetween.
In recent years, the demand for higher power output of internal combustion engines has required increasing the sizes of intake and exhaust valves for the engine and securing a water jacket for cooling of the engine. This results in a decreased space available for installing a spark plug in the engine, thus requiring the spark plug to have a compact (more specifically, slenderized) structure.
Specifically, the threaded portion of the metal shell in a spark plug had an outer diameter of M14 as specified in JIS (Japanese Industrial Standards) in the past; however, the threaded portion is now required to have an outer diameter of M12 or less as specified in JIS.
For example, Japanese Unexamined Utility Model Publication No. H5-55490 discloses such a compact spark plug.
In a slenderized spark plug, the volume of an air pocket, which is the insulation space between an outer surface of the insulator and an inner surface of the metal shell, is accordingly reduced.
The insulator generally includes an intermediate portion and an end portion that includes the end of the insulator protruding from the metal shell. The end portion is thinner than the intermediate portion, and there is provided a frusto-conical shoulder between the two portions. The shoulder engages with an annular seat of the metal shell, which is formed on the inner surface of the metal shell, through a gasket so as to establish a gas-tight seal. Accordingly, the air pocket formed in the spark plug has a range in the lengthwise direction of the insulator from the end of the metal shell to the place where the shoulder of the insulator and the annular seat of the metal shell are in sealing engagement.
When the volume of the air pocket is reduced for slenderizing the spark plug, “inside sparks” can be generated instead of normal sparks to be generated in the spark gap. The inside sparks here denote sparks which creep from the center electrode of the spark plug along the outer surface of the insulator, and jump to the metal shell across the air pocket formed between the insulator and the metal shell. The inside sparks may lead to misfires, thereby resulting in efficiency drop of the engine.
On the other hand, high compression or lean burn engines have recently been used for the purpose of increasing power output and improving fuel economies. However, when the combustion condition of such an engine worsens, carbon and other unburned products will deposit on the outer surface of the end portion of the insulator, thus causing a problem of “carbon fouling”.
When the insulator of a spark plug is fouled with carbon, the above-described inside sparks can be more easily generated in the spark plug. This is because the electrically conductive carbon deposit on the outer surface of the insulator end portion causes a drop in the insulation resistance between the insulator and the metal shell.
Thus one may consider, for the purpose of preventing such a drop in the insulation resistance between the insulator and the metal shell, reducing the volume of the air pocket so as to reduce the amount of carbon and other unburned products to flow into the air pocket and deposit on the outer surface of the insulator end portion.
However, at the same time, the reduced volume of the air pocket will cause, as described previously, generation of inside sparks even when the insulator end portion has not been fouled with carbon.